1. Field of the Invention
The present invention is in the field of fiber optic acoustic sensor arrays wherein light is propagated in the arrays and the effects of acoustic signals on the light returning from the arrays are analyzed to determine the characteristics of the acoustic signals.
2. Description of the Related Art
Fiber optic based acoustic sensors are promising alternatives to conventional electronic sensors. Included among their advantages are a high sensitivity, large dynamic range, light weight, and compact size. The ability to easily multiplex a large number of fiber optic sensors onto common busses also makes fiber optic sensors attractive for large-scale arrays. The recent successful incorporation of multiple small-gain erbium doped fiber amplifiers (EDFAs) into a fiber optic sensor array to increase the number of sensors that can be supported by a single fiber pair has made large-scale fiber optic sensor arrays even more competitive.
For acoustic detection, the fiber optic sensor of choice has been the Mach-Zehnder interferometric sensor. In any interferometric sensor, phase modulation is mapped into an intensity modulation through a raised cosine function. Because of this nonlinear transfer function, a sinusoidal phase modulation will generate higher order harmonics. An interferometer biased at quadrature (interfering beams xcfx80/2 out of phase) has a maximized response at the first order harmonic and a minimized response at the second order harmonic. For this reason, quadrature is the preferred bias point. As the bias point drifts away from quadrature (for example, due to external temperature changes), the response at the first order harmonic decreases and the response at the second order harmonic increases. When the interferometer is biased at 0 or xcfx80 out of phase, the first order harmonic disappears completely. This decreased response at the first order harmonic (resulting from the bias points away from quadrature) is referred to as signal fading.
Because Mach-Zehnder interferometric sensors have an unstable bias point, they are especially susceptible to the signal fading problem just mentioned. In order to overcome signal fading, a demodulation of the returned signal is required. The typical demodulation technique is the Phase-Generated Carrier (PGC) scheme, which requires a path-mismatched Mach-Zehnder interferometric sensor. (See, for example, Anthony Dandridge, et al., Multiplexing of Interferometric Sensors Using Phase Carrier Techniques, Journal of Lightwave Technology, Vol. LT-5, No. 7, July 1987, pp. 947 -952.) This path imbalance also causes the conversion of laser phase noise to intensity noise, which limits the performance of the Mach-Zehnder interferometric sensor arrays at low frequencies and places stringent requirements on the linewidth of the source. This narrow linewidth requirement has slowed the development of amplified Mach-Zehnder interferometric sensor arrays at 1.55 xcexcm.
The Sagnac interferometer has found widespread use in the fiber optic gyroscopes. (See, for example, B. Culshaw, et al., Fibre optic gyroscopes, Journal of Physics E (Scientific Instruments), Vol. 16, No. 1, 1983, pp. 5-15.) It has been proposed that the Sagnac interferometer could be used to detect acoustic waves. (See, for example, E. Udd, Fiber-optic acoustic sensor based on the Sagnac interferometer, Proceedings of the SPIE-The International Society for Optical Engineering, Vol. 425, 1983, pp. 90-91; Kjell Krxc3xa5kenes, et al., Sagnac interferometer for underwater sound detection: noise properties, OPTICS LETTERS, Vol. 14, No. 20, Oct. 15, 1989, pp. 1152-1145; and Sverre Knudsen, et al., An Ultrasonic Fiber-Optic Hydrophone Incorporating a Push-Pull Transducer in a Sagnac Interferometer, JOURNAL OF LIGHTWAVE TECHNOLOGY, Vol. 12, No. 9, September 1994, pp. 1696-1700.) Because of its common-path design, the Sagnac interferometer is reciprocal and therefore has a stable bias point, which eliminates signal fading and prevents the conversion of source phase noise into intensity noise. Therefore, the Sagnac interferometer is immune to the phase noise which limits the Mach-Zehnder interferometric sensors at low frequencies.
One aspect of the present invention is an acoustic sensor that comprises a source of light pulses, a first coupler, a polarization dependent second coupler, an optical delay path and at least one detector. The first coupler couples the light pulses to a first optical path having a first optical length and to an array of sensors. The array of sensors comprises at least a first sensor. The first sensor is in a second optical path having a second optical length different from the first optical length. The polarization dependent second coupler couples light pulses received from the first optical path in a first polarization to the optical delay path and couples light pulses received from the array in a second polarization to the optical delay path. The light pulses coupled to the optical delay path in the first polarization return from the optical delay path to the second coupler in the second polarization. The light pulses coupled to the optical delay path in the second polarization return from the optical delay path to the second coupler in the first polarization. The second coupler couples the light pulses returning to the second coupler from the optical delay path in the first polarization to the first optical path to propagate therein to the first coupler. The second coupler couples light pulses returning to the second coupler from the optical delay path in the second polarization to the array to propagate therein to the first coupler. The first coupler combines the light pulses from the first optical path and the light pulses from the array to cause light pulses traveling equal distances through the first optical path and the array to interfere and to generate a detectable output signal. The detectable output signal varies in response to acoustic energy impinging on the first sensor. The detector detects the detectable output signals to generate a detector output signal responsive to variations in the detectable output signal from the first coupler. Preferably, the array includes a second sensor. The second sensor is in a third optical path having a third optical length different from the first optical length and the second optical length. Also preferably, the polarization dependent second coupler comprises a polarization beam splitter. In preferred embodiments, the optical delay path comprises a length of optical waveguide and a polarization rotating reflector. The reflector causes light incident on the reflector in the first polarization to be reflected as light in the second polarization, and causes light incident on the reflector in the second polarization to be reflected as light in the first polarization. the reflector advantageously comprises a Faraday rotating mirror. In particularly preferred embodiments, the first optical path includes a non-reciprocal phase shifter which causes light propagating through the first optical path in a first direction and light propagating through the first optical path in a second direction to experience a relative phase shift such that light combined in the first coupler has a phase bias. Preferably, In such embodiments, a third optical path is positioned in parallel with the first optical path. One of the first optical path and the third optical path includes an optical delay to cause the first optical path to have an optical path length different from an optical path length of the third optical path, such that light propagating through the first optical path has a propagation time different from a propagation time of light propagating through the second optical path to thereby time multiplex the light pulses. Preferably, the non-reciprocal phase shifter comprises a first Faraday rotator, a quarter-wave plate and a second Faraday rotator, the first Faraday rotator. The quarter-wave plate and the second Faraday rotator are positioned such that light propagating in the first direction passes through the first Faraday rotator, then through the quarter-wave plate, and then through the second Faraday rotator, and such that light propagating in the second direction passes through the second Faraday rotator, then through the quarter-wave plate, and then through the first Faraday rotator. Alternatively, the non-reciprocal phase shifter comprises a first quarter-wave plate, a Faraday rotator, and a second quarter-wave plate. The first quarter-wave plate, the Faraday rotator, and the second quarter-wave plate are positioned such that light propagating in the first direction passes through the first quarter-wave plate, then through the Faraday rotator, and then through the second quarter-wave plate, and such that light propagating in the second direction passes through the second quarter-wave plate, then through the Faraday rotator, and then through the first quarter-wave plate.
Another aspect of the present invention is an acoustic sensor that comprises a source of input light pulses, an array of optical sensors; an optical delay path, an optical detector system; and an input/output system. The input/output system receives the input light pulses and directs a first portion of each light pulse having a first polarization through the array of optical sensors in a first direction, then through the optical delay path, and then to the optical detector system. The input/output system directs a second portion of each light pulse in a second polarization orthogonal to the first polarization through the optical delay path, then through the optical sensor array in a second direction, and then to the optical detector system. The optical detector system receives the light pulses in the first and second polarizations and detects changes in the light pulses caused by perturbations in the optical sensors.
Another aspect of the present invention is a method of detecting acoustic signals. The method comprises generating an input light signal and coupling the input light signal to at least first and second propagation paths to propagate in respective first directions therein. The first and second propagation paths have respective first and second optical lengths. The first and second propagation paths output respective first and second output light portions. The first and second output light portions are output from the first and second propagation paths at differing times in accordance with differences in the first and second optical path lengths. The second output light portion is modulated by an acoustic signal impinging on the second propagation path. The first light portion is coupled to a delay path in a first polarization, and the second light portion is coupled to the delay path in a second polarization. The delay path outputs a first delayed light portion corresponding to the first output light portion. The first delayed light portion has the second polarization. The delay path outputs a second delayed light portion corresponding to the second output light portion. The second delayed light portion has the first polarization. The first and second delayed light portions are coupled to the first and second propagation paths to propagate therein in respective second directions opposite the respective first directions. The first propagation path outputs a first set of return light portions. The first set of return light portions comprise a respective return light portion for each of the first and second delayed light portions. The second propagation path outputs a second set of return light portions. The second set of return light portions comprise a respective return light portion for each of the first and second delayed light portions. The first and second sets of return light portions are coupled to at least one detector. The return light portions in the first and second sets of return light portions result from output light portions and delayed light portions which travel identical optical path lengths and interfere to generate detectable output signals. The method selectively detects the detectable output signals to detect only output signals resulting from interference of light portions which propagated in the first propagation path in either the first direction or the second direction. The detectable output signals vary in response to the acoustic signal impinging on the second propagation path.
Another aspect of the present invention is a sensor that comprises a source of light and a first coupler that couples light to a common path and to a sensing array to propagate in respective first directions therein. The sensing array comprises a plurality of sensing paths. A polarization dependent second coupler couples light from the common path and from the sensing array to a delay path. The second coupler couples only light in a first polarization from the common path to the delay path. The second coupler couples only light in a second polarization from the sensing array to the delay path. The delay path rotates light in the first polarization to the second polarization and rotates light in the second polarization to the first polarization. The second coupler further couples light from the delay path in the first polarization to the common path and couples light from the delay path in the second polarization to the sensing array to propagate in respective second directions therein to the first coupler. The first coupler provides output light responsive to the light propagating in the respective second directions. A detector receives the output light from the first coupler and generates an output signal responsive to interference of light in the first coupler. Preferably, the delay path comprises a length of optical fiber and a polarization rotating reflector. The length of optical fiber is selected to provide an optical delay time. The light propagates through the optical fiber from the second coupler to the reflector. The reflector reflects light into the optical fiber to propagate through the optical fiber to the second coupler. The reflector further rotates light incident in the first polarization to the second polarization and rotates light incident in the second polarization to the first polarization. Preferably, the reflector comprises a Faraday rotating mirror. Also preferably, the polarization dependent second coupler comprises a polarization beam splitter positioned so that the delay path receives the light from a port of the polarization beam splitter and returns light to the port of the polarization beam splitter.